Off-line converter with digital control

ABSTRACT

A circuit protects a power conversion system with a feedback control loop from a fault condition. The circuit has an oscillator having an input for generating a signal with a frequency and a timer connected to the oscillator input and to the feedback control loop. The timer disables the oscillator after a period following the opening of the feedback control loop to protect the power conversion system.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 09/192,871, filedNov. 16, 1998, Now U.S. Pat. No. 6,337,788.

BACKGROUND

The present invention relates to an off-line switched mode controlsystem with fault condition protection.

Quantum leaps in electronic technology have led to the development of“smart” electrical and electronic products. Each of these productsrequires a steady and clean source of power from a power supply. Onepower supply technology called switched mode power supply technologyoperates at a high frequency to achieve small size and high efficiency.In such a switching power supply, an integrated circuit (IC) regulatoris connected in series with the primary winding of a transformer to arectified and filtered alternating current (AC) power line. The energyis transferred from the primary winding through an output secondarywinding to the power supply output in a manner controlled by the ICregulator so as to provide a clean and constant output voltage.Additionally, a third winding called a feedback or bias winding may beused to provide a feedback signal and power to the IC regulator.

The voltage on the feedback winding tracks the output voltage present onthe secondary winding. Thus, when a short occurs on the output of thesecondary winding, the voltage on the feedback winding also goes low.Further, in the event of a short circuit condition, an overloadcondition on the output secondary winding or an open loop condition onthe feedback winding, the regulator circuit responds to such conditionsby delivering maximum power over a period of time. In such cases, theregulator circuit detects that the power supply is short circuited,overloaded at the output or has encountered an open loop condition. Inany of these fault conditions, the regulator circuit goes into a modecalled “auto-restart.” In the auto-restart mode, the regulator circuittries to start the power supply periodically by delivering full powerfor a period of time (greater than needed for start up) and turns offthe power supply for another period of time that is approximately fourto ten times longer. As long as the fault condition is present, theregulator circuit remains in this auto-restart mode limiting the averageoutput power to a safe, low value. When the fault is removed,auto-restart enables the power supply to start-up automatically.

SUMMARY

The invention protects a power supply from fault conditions. The powersupply has an output and a feedback control loop, the feedback controlloop having a feedback signal which cycles periodically when the powersupply operates normally and which remains idle when the power supply isin a fault condition. In a first aspect, the circuit includes aswitching device for controlling power delivered to the output and atimer coupled to the switching device and to the feedback signal. Thetimer disables the switching device to prevent power delivery to theoutput in a first predetermined period after the fault condition exists.

Implementations of the invention include one or more of the following.The timer may enable the switching device to deliver power to the outputafter a second predetermined period. The switching device may bealternately enabled for the first predetermined period and disabled forthe second predetermined period when the fault condition exists. Theswitching device may be enabled upon removal of the fault condition. Theswitching device may be a power transistor. The timer may be a digitalcounter. An oscillator with a predetermined frequency may be coupled tothe counter. The oscillator may have a control input for changing thepredetermined frequency and a first current source coupled to theoscillator control input to generate a first frequency. A second currentsource may be coupled to the oscillator control input to generate asecond frequency. The counter output may be coupled to the fist andsecond current sources. The timer may be a capacitor which is adapted tobe charged at a first rate from a first threshold to a second thresholdto generate a first predetermined period. The capacitor may bedischarged from the second threshold to the first threshold at a secondrate to generate the second predetermined period. The capacitor may alsobe reset to a voltage below the first threshold each time the feedbacksignal cycles. The fault condition includes one or more of an outputoverload fault condition, an output short circuit fault condition and anopen feedback control loop fault condition.

In a second aspect, a method for protecting a power supply having anoutput and a feedback control loop from fault conditions includesreceiving a feedback signal from the feedback control loop, the feedbacksignal being adapted to cycle periodically when the power supplyoperates normally and to remain idle when the power supply is in a faultcondition; timing the feedback signal to detect whether a faultcondition exists in the power supply; and disabling the output after afirst predetermined period after the fault condition is detected.

Implementations of the invention include one or more of the following. Aswitching device may be enabled to deliver power to the output after asecond predetermined period. The switching device may be alternatinglyenabled for the first predetermined period and disabled for the secondpredetermined period. The switching device may be enabled upon removalof the fault condition. The enabling step may enable a power transistor.The timing step includes digitally countering periods of time. A signalmay be generated with a predetermined frequency. The generating stepincludes oscillating at a first frequency and a second frequency. Thesecond frequency may be used when the fault condition exists. The timingstep includes charging a capacitor at a first rate from a firstthreshold to a second threshold to generate a first predeterminedperiod; and discharging the capacitor from the second threshold to thefirst threshold at a second rate to generate a second predeterminedperiod. The capacitor may be reset to a voltage below the firstthreshold each time the feedback signal cycles.

In a third aspect, a circuit for protecting a power supply having anoutput and a feedback control loop from fault conditions includes meansfor receiving a feedback signal from the feedback control loop, thefeedback signal being adapted to cycle periodically when the powersupply operates normally and to remain idle when the power supply is ina fault condition; timing means coupled to the feedback signal to detectwhether a fault condition exists in the power supply system; and meansfor disabling the output after a first predetermined period after thefault condition is detected.

Implementations of the invention include one or more of the following.The circuit includes a means for enabling a switching device to deliverpower to the output after a second predetermined period. A means foralternatingly enabling the switching device for the first predeterminedperiod and disabling the switching device for the second predeterminedperiod when the fault condition exists may be used. The circuit may havea means for enabling the switching device upon removal of the faultcondition. The switching device may be a power transistor. The timingmeans includes a digital counter. The circuit includes means forgenerating a predetermined frequency. The generating means includesmeans for oscillating at a first frequency and a second frequency. Thecircuit may include a means for applying the second frequency when thefault condition exists. The timing means includes a means for charging acapacitor at a first rate from a first threshold to a second thresholdto generate a first predetermined period; and a means for dischargingthe capacitor from the second threshold to the first threshold at asecond rate to generate a second predetermined period. A means forresetting the capacitor to a voltage below the first threshold each timethe feedback signal cycles may be used.

In another aspect, a fault protected power supply includes a regulatorcoupled to a transformer having a primary winding. The transformer has asecondary winding coupled to a secondary output. The regulator receivesa feedback signal from the secondary output which cycles periodicallywhen the power supply operates normally and which remains idle when thepower supply is in a fault condition. The power supply includes aswitching device coupled to the primary winding of the transformer forcontrolling power delivered to the secondary output; an oscillator forgenerating a signal with a predetermined frequency; and a timer coupledto the oscillator and to the feedback signal, the timer disabling theswitching device after a predetermined period of existence of a faultcondition.

Implementations of the invention include one or more of the following.The power supply includes a means for changing the frequency of theoscillator. The timer alternatively enables and disables the switchingmeans when the fault condition is present.

In another aspect, a method protects a power supply having a regulatorcoupled to a transformer having primary winding, the transformer havinga secondary winding coupled to a secondary output, the regulatorreceiving a feedback signal from the secondary output which cyclesperiodically when the power supply operates normally and which remainsidle when the power supply is in a fault condition. The method includescontrolling power delivered to the secondary output using a switchingdevice; generating an oscillating signal with a predetermined frequency;and timing the feedback signal with the oscillating signal and disablingthe switching device after a predetermined period of existence of afault condition.

Implementations of the invention include one or more of the following.The method includes changing the frequency of the oscillating signal.The method also includes alternatingly enabling and disabling theswitching device when the fault condition is present.

In another aspect, a fault protected power supply has a regulatorcoupled to a transformer having a primary winding, the transformerhaving a secondary winding coupled to the secondary output. Theregulator receives a feedback signal from the secondary output whichcycles periodically when the power supply operates normally and whichremains idle when the power supply is in a fault condition. The powersupply includes a switching device coupled to the primary winding of thetransformer for controlling the power delivered to the secondary output;a capacitor; means for charging the capacitor at a first rate from afirst threshold to a second threshold to generate a first predeterminedperiod and discharging the capacitor from the second threshold to firstthreshold at a second rate to generate a second predetermined period;and means coupled to the switching device, the capacitor and thefeedback signal for alternately enabling the switching device duringfirst predetermined period and disabling the switching device during thesecond predetermined period in the presence of a fault condition.

In yet another aspect, a method protects a power supply having aregulator coupled to a transformer having a primary winding. Thetransformer has a secondary winding coupled to a secondary output. Theregulator receives a feedback signal from the secondary output whichcycles periodically when the power supply operates normally and whichremains idle when the power supply is in a fault condition. The methodincludes controlling power delivered to the secondary output using aswitching device; charging a capacitor at a first rate from a firstthreshold to a second threshold to generate a first predeterminedperiod; discharging the capacitor from the second threshold to firstthreshold at a second rate to generate a second predetermined period;and alternatingly enabling the switching device during the firstpredetermined period and disabling the switching device during thesecond predetermined period in the presence of a fault condition.

Advantages of the invention include one or more of the following. Theinvention protects the switched mode controller and associatedcomponents such as the diode and the transformer from various faultconditions. The feedback winding is not necessary. The protection isprovided using a minimum number of components. Further, the power supplyproperly shuts down when it encounters a fault condition andautomatically returns to an operating condition when the fault conditionis removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a fault condition protectiondevice of the invention.

FIG. 2 is a plot illustrating the operation of the device of FIG. 1.

FIG. 3 is a schematic illustration of a second embodiment of the faultcondition protection device.

FIG. 4 is a plot illustrating the operation of the device of FIG 2.

FIG. 5 is a schematic illustration of a switched mode power supply inaccordance with the present invention.

DESCRIPTION

Referring now to FIG. 1, a fault-protection circuit 200 is shown. Thecircuit 200 has a primary oscillator 111 which is connected to a counter202. The counter 202 can be reset by a feedback signal which clearsregisters Q8-Q13 of counter 202. The feedback signal is explained inmore detail below.

An inverter 204 receives the 13-th bit output of counter 202. The outputof inverter 204 is provided to an AND-gate 206 whose other input isconnected to a switching signal. The switching signal is derived fromthe oscillator 111 output and the feedback signal. This switching signalcycles periodically when the power supply operates normally. Theswitching signal is idled when the power supply encounters a faultcondition. The output of AND-gate 206 in turn is provided to the gate ofa switching transistor 208. Counter 202 eventually causes an AND-gate206 to shut-off switching transistor 208 and to perform auto-restart.

Turning now to oscillator 111, a current source 122 generates a currentI from a supply voltage 120. The output of current source 122 isconnected to the source of a p-channel MOSFET transistor 125, whosedrain is connected to a node 123. Also connected to the node 123 througha p-channel MOSFET 182 is a second current source 184. Current source184 can supply current which is ¼ of the current I. The drain oftransistor 182 is also connected to node 123. The gate of transistor 182is driven by an inverter 180, whose input is connected to the gate oftransistor 125 and to the counter output Q13.

The node 123 is connected to the sources of p-channel MOSFET transistors126 and 132. The drain of MOSFET transistor 126 is connected to thedrain of an n-channel MOSFET transistor 128. The source of transistor128 is grounded, while the gate of transistor 128 is connected to itsdrain. The gate of transistor 128 is also connected to the gate of ann-channel MOSFET transistor 130. The source of transistor 130 isgrounded, while the drain of transistor 130 is connected to the drain oftransistor 132 at a node 131. Transistors 126, 128, 130 and 132 form adifferential switch. The input of inverter 124 and the gate oftransistor 132 are driven by a hysteresis comparator 136. Output ofinverter 124 drives the gate of MOSFET transistor 126. Comparator 136has an input which is connected to node 131 and to a capacitor 134. Theother node of the capacitor is connected to ground. In combination,transistors 126, 128, 130 and 132, capacitor 134, inverter 124 andhysteresis comparator 136 and current source 122 form an oscillator. Theoutput of hysteresis comparator 136 is provided as an oscillator outputand is also used to drive the clock input of counter 202.

During operation, the feedback signal periodically pulses between a lowstate and a high state depending on the amount of power required on asecondary winding 922 (FIG. 5). Every time the feedback signal is low,the feedback signal resets a counter whose states are reflected byoutputs Q8-Q13 of counter 202. The resetting of the counter associatedwith outputs Q8-Q13 thus occurs regularly when no fault is present inthe power supply. The cycling of the feedback signal constantly clearsthe output bit Q13 such that the power transistor 208 is controlled bythe switching signal when no fault is present. However, in the event ofa fault condition, the feedback signal remains high for a sufficientlylong time such that the counter associated with output bits Q8-Q13 hasenough time to increment output bit Q13. The setting of the output bitQ13 causes inverter 204 output to go low and thus causes the output ofAND-gate 206 to be deasserted. The deassertion of AND-gate 206 in turndisables switching transistor 208. Also, when the counter output Q13goes high transistor 125 turns off to isolate primary current source 122from node 123. This turns on the transistor 182 via inverter 180, thusallowing the ¼ I current to flow from the secondary current source 184to node 123. The state change of the counter output Q13 causes theoscillator to switch at one-fourth of its normal frequency to achieveabout 20% on time and 80% off time. This operation reduces the powerdelivered by the power supply under a fault condition as well as avoidsthe possibility of damage to the regulator device and other power supplycomponents such as the output diode or the transformer (not shown).

FIG. 2 shows a timing diagram for the device of FIG. 1. The timingdiagram of FIG. 2 shows three periods: 211, 213 and 215. Period 211 isnormal operation with the feedback signal going “low” more often than apredetermined count such as approximately 4096 clock cycles, therebyresetting the Auto Restart Counter before it counts up to 4096.

In Period 213, the feedback signal has been “high” for 4096 continuousclock cycles due to a fault condition such as an output overload orshort, so the circuit of FIG. 1 goes into the auto-restart mode. Theoscillator frequency is divided by four and switching transistor 208 hasbeen inhibited from switching, remaining in its off state. After 4096clock cycles, switching transistor 208 is activated and the oscillatorfrequency switches back to normal frequency. This sequence will repeatitself as long as the feedback signal stays “high.”

In Period 215, the overload condition or the short condition on theoutput of the power supply is removed and the feedback signal goes low,indicating the power supply output is in regulation. The circuit is nowin normal operation with the feedback signal going “low” at least onceevery 4096 clock cycles. It is to be noted that the auto-restartcapability as been described may not be used in all applications.Particularly, certain applications may disable the power regulator afterdetecting a fault condition and the disabling of the power regulator maycontinue until a user resets the power regulator, or until AC power iscycled OFF and then ON to the power regulator.

FIG. 3 shows an analog auto restart circuit. A current source 525produces a fixed magnitude current 530. Fixed magnitude current 530 isfed into first transistor 535 and mirrored to transistors 540 and 545.Third transistor 545 is connected to a capacitor 550 via transistor 595.Transistor 600 is also connected to the capacitor 550. Transistor 600 iscontrolled by the feedback signal provided to inverter 605 whose outputdrives the gate of the transistor 600. Node 400 is generated by thecharging and discharging of capacitor 550. Capacitor 550 has arelatively low capacitance which allows for integration on a monolithicchip in one embodiment of the IC regulator of the invention. Node 400 isprovided to a hysteresis comparator 560 which compares its input with alower limit of about 1.5 volts and an upper limit of about 4.5 volts.The output of comparator 560 is provided to the gates of transistors 585and 595. AND-gate 570 receives at one input the output of comparator560. AND-gate 570 enables switching transistor 572 to turn on and off.AND-gate 570 receives at a second input a switching signal whichmodulates the regulator output.

In operation, after the feedback signal goes high, capacitor 550 beginsto charge from a level below 1.5 volts to an upper threshold of about4.5 volts. Upon reaching 4.5 volts, the output of comparator 560switches and discharges the capacitor 550 through transistors 545 and595. Node 400 then switches between the upper threshold of about 4.5volts and the lower threshold of about 1.5 volts.

Signal 401 output of comparator 560 will be high until node 400 exceedsthe upper threshold limit. When signal 400 is high, p-channeltransistors 585 and 595 are turned off. By turning off transistors 585and 595, current can flow into and steadily charge capacitor 550 andincrease the magnitude of node 400. The current that flows intocapacitor 550 is derived from current source 525 because the currentthrough transistor 590 is mirrored from transistor 580, which current isderived from transistor 540.

Referring to FIGS. 3 and 4, in period 600 feedback signal 402 isswitching and the system is in normal operation with switchingtransistor 572 controlled by the switching signal. At the end of period600 a fault condition has been detected and the feedback signal stayshigh for an extended period of time (period 601). In period 601,transistor 600 turns off, allowing capacitor 550 to be charged bycurrent source 590. When the voltage on node 400 has reached the secondthreshold, the output 401 of comparator 560 goes low, disabling theswitching transistor 572. Capacitor 550 will be discharged to the firstthreshold by current source 545 with switching transistor 572 disabled.This mode of oscillation continues until the feedback signal goes lowagain, indicating that the fault condition no longer exists. When thefeedback signal 402 at the end of period 601 goes low, transistor 600turns on and discharges capacitor 550 to a voltage below the firstthreshold. Comparator 560 output will go high and enable the switchingsignal to control the switching transistor 572. In period 602, thesystem has returned to normal operation with the feedback signal 402going low at least once during a defined time period indicating that theregulator circuit is in regulation.

Referring now to FIG. 5, a switched mode power supply is shown. Directcurrent (DC) input voltage is provided to a Zener diode 912 which isconnected to a diode 914. The diodes 912-914 together are connected inseries across a primary winding of a transformer 920. A secondarywinding 922 is magnetically coupled to the primary winding oftransformer 920. One terminal of the secondary winding 922 is connectedto a diode 930, whose output is provided to a capacitor 932. Thejunction between diode 930 and capacitor 932 is the positive terminal ofthe regulated output. The other terminal of capacitor 932 is connectedto a second terminal of the secondary winding and is the negativeterminal of the regulated output. A Zener diode 934 is connected to thepositive terminal of the regulated output. The other end of Zener diode934 is connected to a first end of a light emitting diode in anopto-isolator 944. A second end of the light-emitting diode is connectedto the negative terminal of the regulated output. A resistor 936 isconnected between the negative terminal of the regulated output and thefirst end of the light-emitting diode of opto-isolator 944. Thecollector of the opto-isolator 944 is connected to current source 172.The output of current source 172 is provided to the switching regulatorlogic 800.

Connected to the second primary winding terminal is the power transistor208. Power transistor 208 is driven by AND gate 206 which is connectedto inverter 204 and switching regulator logic 800. Switching regulatorlogic 800 receives a clock signal 101 from an oscillator 111. A counter202 also receives the clock signal 101 from the primary oscillator 111.The output of counter 202, Q13, is used to switch in the current source184 to supply current in lieu of the current source 122 when Q13 ishigh.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

What is claimed is:
 1. A circuit for protecting a power supply fromfault conditions, comprising: a switching device coupled to receive aswitching signal and operable to control power delivered to an output ofthe power supply; a feedback input for receiving a feedback signalrepresentative of the output of the power supply, the feedback signalcycling between a first state and a second state when the power supplyoperates normally and not cycling between the first and second stateswhen the power supply is in a fault condition, the switching signalcycling separately from the cycling of the feedback signal; and a timercoupled to the switching device and the feedback input, the timeroperable to disable the switching device to prevent power delivery tothe output after detection of a fault condition.
 2. The circuit of claim1 wherein the feedback signal cycles periodically between the first andsecond states when the power supply operates normally.
 3. The circuit ofclaim 1 wherein the existence of a fault condition is indicated by thefeedback signal maintaining either the first state or the second statefor at least a predetermined period.
 4. The circuit of claim 1 whereinthe timer disables the switching device after a first period ofexistence of the fault condition and wherein the timer enables theswitching device after a second period.
 5. The circuit of claim 4wherein the switching device is alternately enabled for the first periodand disabled for the second period when the fault condition exists. 6.The circuit of claim 5 wherein the switching device is enabled uponremoval of the fault condition.
 7. The circuit of claim 6 furthercomprising a capacitor that is charged at a first rate from a firstthreshold to a second threshold to generate the first period, thecapacitor further being discharged from the second threshold to thefirst threshold at a second rate to generate the second period.
 8. Thecircuit of claim 7 wherein the capacitor is discharged to a voltagebelow the first threshold each time the feedback signal changes state.9. The circuit of claim 7 wherein the capacitor is discharged to avoltage below the first threshold each time the feedback signal entersthe first state.
 10. The circuit of claim 1 wherein the switching deviceis a power transistor.
 11. The circuit of claim 1 wherein the timercomprises a digital counter.
 12. The circuit of claim 11 furthercomprising an oscillator coupled to the counter, the oscillator having apredetermined frequency.
 13. The circuit of claim 12, wherein theoscillator has a control input for changing the predetermined frequency,further comprising a first current source coupled to the oscillatorcontrol input to generate a first frequency.
 14. The circuit of claim 13further comprising a second current source coupled to the oscillatorcontrol input to generate a second frequency.
 15. The circuit of claim14 wherein the counter has an output coupled to the first and secondcurrent sources.
 16. The circuit of claim 1 wherein the fault conditionincludes one or more of an output overload fault condition, an outputshort circuit fault condition and an open feedback control loop faultcondition.
 17. A method for protecting a power supply from faultconditions, comprising: in response to a switching signal, enabling ordisabling the delivery of power to an output of the power supply;receiving a feedback signal representative of the output of the powersupply, the feedback signal cycling between a first state and a secondstate when the power supply operates normally and not cycling betweenthe first and second states when the power supply is in a faultcondition, the switching signal cycling separately from the cycling ofthe feedback signal; timing the feedback signal to detect whether afault condition exists in the power supply; and preventing the switchingsignal from enabling power delivery to the output in response to thedetection of a fault condition.
 18. The method of claim 17 wherein thefeedback signal cycles periodically between the first and second stateswhen the power supply operates normally.
 19. The method of claim 17wherein the existence of a fault condition is indicated by the feedbacksignal maintaining either the first state or the second state for atleast a predetermined period.
 20. The method of claim 17 wherein theswitching signal is prevented from enabling power delivery to the outputafter a first period of existence of the fault condition and theswitching signal is allowed to enable power delivery after a secondperiod.
 21. The method of claim 20, further comprising altematinglyallowing the switching signal to enable power delivery to the output forthe first period and preventing the switching signal from enabling powerdelivery to the output for the second period.
 22. The method of claim 21further comprising allowing the switching signal to enable powerdelivery to the output upon removal of the fault condition.
 23. Themethod of claim 17 wherein the timing step includes counting periods oftime digitally.
 24. A circuit for protecting a power supply from faultconditions, comprising: switching means coupled to receive a switchingsignal for controlling the delivery of power to an output of the powersupply; a feedback input for receiving a feedback signal representativeof the output of the power supply, the feedback signal cycling between afirst state and a second state when the power supply operates normallyand not cycling between the first and second states when the powersupply is in a fault condition, the switching signal cycling separatelyfrom the cycling of the feedback signal; timing means coupled to theswitching means and the feedback input for detecting a fault conditionin the power supply and means for disabling the switching means inresponse to the detection of a fault condition.
 25. The circuit of claim24 wherein the feedback signal cycles periodically between the first andsecond states when the power supply operates normally.
 26. The circuitof claim 24 wherein the existence of a fault condition is indicated bythe feedback signal maintaining either the first state or the secondstate for at least a predetermined period.
 27. The circuit of claim 24wherein the switching means is disabled after a first period ofexistence of the fault condition and further comprising means forenabling the switching means after a second period.
 28. The circuit ofclaim 27, further comprising means for alternatingly enabling theswitching means for the first period and disabling the switching meansfor the second period.
 29. The circuit of claim 28 further comprisingmeans for enabling the switching means upon removal of the faultcondition.
 30. The circuit of claim 24 wherein the timing means includesa digital counter.
 31. The circuit of claim 30 further comprising meansfor oscillating at a first frequency when the switching means is enabledand at a second frequency when the switching means is disabled.
 32. Thecircuit of claim 24 further comprising: means for charging a capacitorat a first rate from a first threshold to a second threshold to generatethe first period; and means for discharging the capacitor from thesecond threshold to the first threshold at a second rate to generate thesecond period.
 33. The circuit of claim 32 further comprising means fordischarging the capacitor to a voltage below the first threshold eachtime the feedback signal changes state.
 34. The circuit of claim 32further comprising means for discharging the capacitor to a voltagebelow the first threshold each time the feedback signal enters the firststate.
 35. A fault protected power supply, comprising: a energy transferelement having a input winding and an output winding, the output windingcoupled to a secondary output; a switching device coupled to the inputwinding of the energy transfer element for controlling delivery of powerto the secondary output in response to a switching signal; an oscillatorfor generating a signal with a predetermined frequency; a feedbacksignal representative of the secondary output, the feedback signalcycling between a first state and a second state when the power supplyoperates normally and not cycling between the first and second stateswhen the power supply is in a fault condition, the switching signalcycling separately from the cycling of the feedback signal; and a timercoupled to the oscillator and coupled to receive the feedback signal,the timer operable to disable the switching device when the feedbacksignal indicates that the power supply is in a fault condition.
 36. Thepower supply of claim 35 further comprising means for changing thefrequency of the oscillator.
 37. The power supply of claim 35 whereinthe timer is further operable to alternatively enable and disable theswitching device when the fault condition is present.
 38. A method forprotecting a power supply, comprising: in response to a switchingsignal, enabling or disabling power delivery to a secondary output ofthe power supply; generating an oscillating signal with a predeterminedfrequency; timing a feedback signal representative of the secondaryoutput with the oscillating signal to detect a fault condition, thefeedback signal cycling between a first state and a second state whenthe power supply operates normally and not cycling between the first andsecond states when the power supply is in a fault condition, theswitching signal cycling separately from the cycling of the feedbacksignal; and in response to the detection of a fault condition,preventing the switching signal from enabling power delivery to thesecondary output.
 39. The method of claim 38 further comprising: whenthe fault condition is present, alternately allowing the switchingsignal to enable power delivery and preventing the switching signal fromenabling power delivery.
 40. An automatically restartable power supply,comprising: a energy transfer element having a input winding and anoutput winding, the output winding coupled to a secondary output; aswitching device coupled to the input winding of the energy transferelement for controlling the delivery of power to the secondary output inresponse to a switching signal; a feedback signal representative of thesecondary output, the feedback signal cycling between a first state anda second state when the power supply operates normally and not cyclingbetween the first and second states when the power supply is in a faultcondition, the switching signal cycling separately from the cycling ofthe feedback signal; a capacitor; means for charging the capacitor at afirst rate from a first threshold to a second threshold to generate afirst period and discharging the capacitor from the second threshold tothe first threshold at a second rate to generate a second period; meanscoupled to the switching device, the capacitor and the feedback signal,for alternately enabling the switching device during the first periodand disabling the switching device during the second period when thefeedback signal indicates that the power supply is in a fault condition;and means for enabling the switching device when the feedback signalindicates the power supply is no longer in a fault condition.
 41. Amethod for automatically restarting a power supply after the occurrenceof a fault, comprising: in response to a switching signal, enabling ordisabling the delivery of power to a secondary output of the powersupply; receiving a feedback signal representative of the secondaryoutput, the feedback signal cycling between a first state and a secondstate when the power supply operates normally and not cycling betweenthe first and second states when the power supply is in a faultcondition, the switching signal cycling separately from the cycling ofthe feedback signal; generating a first period; generating a secondperiod; when the feedback signal indicates that the power supply is in afault condition, alternately allowing the switching signal to enablepower delivery to the secondary output during the first period andpreventing the switching signal from enabling power delivery to thesecondary output during the second period; and allowing the switchingsignal to enable power delivery to the secondary output when thefeedback signal indicates the power supply is no longer in a faultcondition.